PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



found in the upper atmosphere are of 

 terrestrial origin. Radon measure- 

 ments in the southern hemisphere 

 south of the North Atlantic trade 

 winds can provide valuable informa- 

 tion on cross-equatorial flow and the 

 flow of air across the Intertropic 

 Convergence Zone. 



In one aircraft expedition made by 

 a U.S. research team flying between 

 Miami and Dakar, a high correlation 

 was found between haze and radon 

 activity. This relationship between 

 dust and radon activity was substan- 

 tiated in further aircraft flights made 

 near Barbados in I^b® and by some 

 measurements made on board the 

 US5 Discoverer the same year. Radon 

 was also measured south of the equa- 

 tor on the flight. More such flights 

 and expeditions are needed to expand 

 our fragmented knowledge of dust 

 transport. 



LIDAR — Another indirect method 

 for estimating the vertical distribu- 

 tion of dust is with LIDAR, which 

 measures the back-scatter from a 

 laser beam. However, back-scatter 



measurements are highly dependent 

 on particle size and are extremely 

 difficult to interpret in terms of dust 

 distribution without supporting data 

 to accompany them. 



Turbidity Measurement -- More 

 useful than LIDAR in the study of 

 dust is the measurement of turbidity 

 from photometric measurements of 

 skylight distribution and spectral at- 

 tenuation of solar radiation. These 

 turbidity measurements can also be 

 compared with atmospheric back-scat- 

 ter and albedo as determined from 

 satellites. Atmospheric dust over the 

 tropical Atlantic can have an im- 

 portant effect on the energy balance 

 of the tropics and, consequently, on 

 the global circulation. Since the at- 

 mospheric turbiditv is a function of 

 the aerosol content of the air, the 

 total incoming and outgoing radia- 

 tion and the changes in absorptivity 

 and emissivity on the vertical can af- 

 fect the heating and the convective 

 instability of the trade winds. There 

 is some evidence that the growing 

 pollution over the earth during the 

 past few decades has resulted in an 



increase in atmospheric turbidity and 

 a slight decline in worldwide tem- 

 perature. An increase in turbidity at 

 low latitudes can effect a decrease in 

 worldwide temperature and a slowing 

 down of the general circulation of the 

 whole earth. At present there is 

 some question as to the cause of the 

 turbidity increase over the years. It 

 may actually be due to natural causes 

 such as volcanic eruptions or changes 

 in dust content of the air rather than 

 to industrial pollution. Since sig- 

 nificant changes in dust loading from 

 year to year do occur in the Atlantic 

 trade winds (the amount of dust 

 reaching Barbados in the summer of 

 1969 was double that in the previous 

 four years of record), it would there- 

 fore be useful to measure turbidity in 

 the Atlantic trade-wind area on a 

 yearly basis in order to determine the 

 natural fluctuation in the components 

 of the radiation balance there. 



African dust may thus influence 

 tropical storm development indirectly, 

 by means of its capacity to alter the 

 long-term thermodynamics of the 

 tropical environment. 



194 



